1. Field of the Invention
The present invention relates to a method of recovering sparingly water-soluble or water-insoluble volatile organic matter from a gas or gas mixture containing the organic matter. More particularly, the present invention relates to a method of effectively recovering sparingly water-soluble or water-insoluble volatile organic matter from a gas or a gas mixture, such as air or industrial off-gas, having the organic matter diffused thereinto in the form of vapor, mist and/or small droplets even at a relatively low concentration according to a simple two-step capturing treatment wherein the gas or gas mixture is first brought into gas-liquid contact with a specific aqueous capturing system and the aqueous capturing system is then brought into liquid-liquid contact with an organic liquid which is immiscible with the aqueous capturing system and has a higher distribution ratio of the organic matter than that aqueous capturing system.
2. Description of the Prior Art
In recent years, a large amount of volatile organic matter such as fuels and various kinds of organic solvents are diffused into the air with the rapid development of industry and transportation means causing air pollution, including the generation of the so-called photochemical smog and the like physiologically harmful atmosphere, which is now taken up as one of the big social problems to be solved. In general, such volatile organic matter are liquids sparingly soluble or insoluble in water and are unavoidably exhausted and diffused into the air at a relatively low concentration during operations using such organic matter. Nowadays, such volatile organic matter is consumed or handled as fuels or solvents in various fields of industry, such as the petroleum depot, petroleum refining plants, painting factories, printing factories, rubber-processing factories and general chemical plants, and even in small scale shops such as gasoline stands existing everywhere in cities. Diffusion of such volatile organic matter into the air by evaporation and the like phenomena is indeed considerable, especially in the fields of petrochemistry and various chemical industries, such as the fields of manufacturing and using printing inks and paints, where large amounts of volatile organic solvents and vehicles are consumed.
The existence of a large amount of volatile organic matters at a relatively low concentration in the air, ventilated exhaust and off-gas from the above-mentioned factories and plants creates on the one hand a big social problem of air-pollution harmful to public health, while on the other hand presenting an interesting problem of recovering such organic matter which cannot be overlooked from the viewpoint of effective utilization and saving of resources. Proposed hitherto for recovering volatile organic matter diffused in a gas or gas mixture are an adsorption method wherein the organic matter is adsorbed onto a solid adsorbent and an absorption method wherein the volatile organic matter is absorbed in a liquid absorbent. In these methods, however, a satisfactory adsorption or absorption of the volatile organic matter can be expected only within a short period of time, usually at the initial stage of the operation, because of saturation in adsorption or absorption equilibrium. To say it in another way, the adsorption or absorption capacity of the adsorbent or absorbent decreases rapidly as the operation proceeds. For maintaining a constant high level of adsorption or absorption in these methods, therefore, exchange of the adsorbent or absorbent is required frequently throughout the operation. In a petroleum depot, for example, the amount of volatile fuels actually lost by evaporation in the course of filling or emptying oil tanks or containers is indeed enormous. When the air expelled from the tanks or containers is treated according to the dry adsorption method to recover the volatile fuels, the adsorbent will soon be put in a saturated state so that a steady highly efficient adsorption operation cannot be realized unless the adsorbent is exchanged frequently or an adsorption-desorption treatment carried out frequently. Thus, the dry adsorption method is hardly utilized due to the above disadvantages.
In the absorption method, on the other hand, white oil, spindle oil or the like oil is used as a liquid absorbent. In general, water-insoluble volatile organic matter such as gasoline and toluene are neutral substances and so cannot be absorbed in the acidic or alkaline liquids usually employed for absorption treatments as a liquid absorbent. In addition, water itself cannot be used as the liquid absorbent for such hydrophobic organic matter since, by definition, the organic matter, in principle, is sparingly soluble or substantially insoluble in water. Thus, the less volatile organic liquid materials which are miscible with the water-insoluble volatile organic materials are generally used as a liquid absorbent for capturing the organic matter. In the case of using white oil or spindle oil in such a wet absorption method, however, the phenomenon of saturation is unavoidable because of absorption equilibrium. When a gas or gas mixture containing such volatile organic matter is continuously passed through the same liquid absorbent, the amount of the organic matter absorbed becomes gradually smaller as it approaches the absorption equilibrium. Thus, it is also quite impossible, in the wet absorption method, to perform efficient absorption of the volatile organic matter continuously and economically. For the above reasons, no industrially practical method has been proposed hitherto for effectively recovering volatile organic matter from a gas or gas mixture for the dual purpose of preventing the atmosphere from physiologically harmful pollution and for economically utilizing the organic matter for repeated use. Thus, there is still a great demand for developing a method for effectively recovering volatile organic matter in a simple and economical manner from a gas or gas mixture having the organic matter diffused thereinto.